Insulation where floor joists overhang foundation

[Dana]

I've seen worse houses than yours in MA- trust me! I recently saw a raised-ranch just outside of 128 with similar issues at the cantilevers, but worse yet, it only had R8 econobatts in the 2x8 ceiling joists below the attic half, and who knows what (if anything) is between the 2x6 rafters in the cathedralized ceiling half! (I advised my friend to not buy that one.)

If the facers on the foam are in contact with adjacent layers it doesn't matter which way you orient the facers.

Kraft facers on batts are not vapor barriers, but batts need air barriers in contact with both sides of the batt. As kraft facers acquire moisture they become fairly vapor open (5+ perms), whereas when dry they're about 0.5 perms. A 3/4" plywood or OSB subfloor is already a pretty good vapor retarder, between 0.5-1 perms. As long as the cladding on the under side of the cantilever is more than 0.5 perms there isn't a vapor diffusion path to be concerned about, but if there isn't an air barrier where the joists cross the foundation there can be a fairly significant air-convection movement of moisture to the band joist and the bottom cladding of the cantilever.

Ideally the plumbing would be all COMPLETELY inside the pressure & thermal boundary of the house. But given that the ideal isn't possible without some re-plumbing, how much space is there between the pipes and the bottom of the cantilever bay, and how far from the band joist?

The stack up matters, since foil facers on rigid polyisocyanurate are true vapor barriers. It's not totally clear where you intend to place the foam board relative to the batts, or the rest. It sounds as if you're going to apply the rigid foam to the underside of the subfloor over the cantilever, and fill the rest with R30 batt(?). Or are you only talking about cut'n'cobbled Tuff-R as the air barriers where the joists cross the foundation sill? Or is it something else?

 

[miracj]

If the facers on the foam are in contact with adjacent layers it doesn't matter which way you orient the facers.

Kraft facers on batts are not vapor barriers, but batts need air barriers in contact with both sides of the batt. As kraft facers acquire moisture they become fairly vapor open (5+ perms), whereas when dry they're about 0.5 perms. A 3/4" plywood or OSB subfloor is already a pretty good vapor retarder, between 0.5-1 perms. As long as the cladding on the under side of the cantilever is more than 0.5 perms there isn't a vapor diffusion path to be concerned about, but if there isn't an air barrier where the joists cross the foundation there can be a fairly significant air-convection movement of moisture to the band joist and the bottom cladding of the cantilever.

Ideally the plumbing would be all COMPLETELY inside the pressure & thermal boundary of the house. But given that the ideal isn't possible without some re-plumbing, how much space is there between the pipes and the bottom of the cantilever bay, and how far from the band joist?

The stack up matters, since foil facers on rigid polyisocyanurate are true vapor barriers. It's not totally clear where you intend to place the foam board relative to the batts, or the rest. It sounds as if you're going to apply the rigid foam to the underside of the subfloor over the cantilever, and fill the rest with R30 batt(?). Or are you only talking about cut'n'cobbled Tuff-R as the air barriers where the joists cross the foundation sill? Or is it something else?

I've attached a picture to show my setup and my intention for the batts and solid foam. There may be space between the brick and the stud wall, but I am not sure (but would think there should be). The floor I believe is 3/4" plywood with 1/2" OSB on top of that (but I am not positive).

My intention is to use the "cut'n'cobbled Tuff-R as the air barriers where the joists cross the foundation sill" as you said, then to cover the underside of the subfloor between that piece and the Header/band Joist, and spray foam the edges to seal the . The rest of the joist would be filled with the R30 Batt, and covered by the vinyl siding underneath.

The pipes only come down through the subfloor about an inch or two from the bottom of the subfloor near the the edge of the cantilevered just before the inside stud wall and then go through the joist to go to the next hot water radiator along the outside wall.

I hope that is to code (but wouldn't surprise if it wasn't) in terms of where they drilled the joist hole.

My gut feeling that says that even if they always have hot water in them running in the winter, it would be better to cut the foam so they are on the warm side of it (which I could do).

 

[Sponsor]

 

[Dana]

The detail for the cantilever overhang itself is just fine. As drawn there appears to be is some sort of air barrier (OSB? plywood?) between the vinyl siding underside of the cantilever and the R30 which would be correct, not just vinyl siding tacked to the joists. Air sealing that air barrier to the framing is worthwhile too. The only restriction on the bottom side air barrier is that it be at least semi-permeable to water vapor when the entrained air in the R30 is 70% RH or higher. Both plywood and OSB would be in the 5 perms range under those conditions, which is fine.

As long as the bulk of the R-value is exterior to the plumbing it's fine to just leave it there. Air sealing the plumbing penetrations is still important though. Due the heating/cooling dimension changes of heating system plumbing it's better to use a flexible type of air sealant, not standard formula expanding polyurethane can foams (Window & Door versions are probably OK, as long as you're not running 215F steam in that plumbing, which you're not). I don't believe there are code restrictions for running heating system plumbing in exterior walls/floors but it's not a big deal if it's located less than half the R-value out from the interior. With R13 polyiso on the interior and R25 fiberglass to the exterior it's about 1/3 of the way through the R. Even during an extended power outage when the indoor temperatures dropped to 45F indoors it would take an extended periods of 0F or lower outdoor temps for that pipe to actually freeze. That's just not going to happen in Boston.

Code R values for zone 5 (all of MA) for exposed floors is R30, and you'll be a bit over that. The compressed R30 would be performing at about R25 due to the the compression, but that's more than made up by the R12-ish polyiso. If you wanted to you could skip the polyiso under the subfloor and just air seal the subfloor to the framing and it would still meet code. The subfloor itself is a "smart" vapor retarder, sufficiently low permeance to protect the band joist and whatever you install as the bottom air barrier from interior moisture drives.

There appears to be a thermal bridge where the joists go over the 2x4 & brick wall. A cut'n' cobbled slab o' foam that extended over the top plate of the 2x4 & brick foam-sealed to the joists would be a good idea, along with moving your polyiso air barriers toward the interior, so they are roughly co-planar with the wallboard of the studwall. You'd then stuff the R30s so that they extend over the top of that polyiso slab over bricks & studwall filling the cavity.

As drawn there is no foundation insulation either, which is another major heat leak. A typical 8-10" poured foundation is about R1. Every square foot of above grade exposed concrete it losing more heat than 10 square feet of the brick & insulated 2x4 wall above. Even at a 50F basement temp that's a huge heat loss. Code min for new construction is R15 continuous insulation, but you could get the same performance out of 1" polyiso trapped to the foundation with a 2x4/R13 studwall and NO interior side vapor barriers. Ideally the 2x4 pony wall above the foundation would NOT have the polyethylene vapor barrier indicated in the drawing, but it's not worth ripping it out- the brick is protected from direct wetting by the overhang, and it will dry just fine toward the exterior. But for the sub grade section it's critical that there be no interior side vapor retarders (latex paint on wallboard is fine, but not vinyl or foil wallpapers.)

 

[miracj]

As drawn there appears to be is some sort of air barrier (OSB? plywood?) between the vinyl siding underside of the cantilever and the R30 which would be correct, not just vinyl siding tacked to the joists. Air sealing that air barrier to the framing is worthwhile too. The only restriction on the bottom side air barrier is that it be at least semi-permeable to water vapor when the entrained air in the R30 is 70% RH or higher. Both plywood and OSB would be in the 5 perms range under those conditions, which is fine.

Unfortunately, it is only the vinyl siding that is there (stuck into guides at the wall and Header/band joist). If I were to install some air barrier there, I probably would use a minimal thickness and place the vinyl siding on those, as there is not much of a drip edge (if any) on the front. Would pressure treated wood be best for this underside?

Even during an extended power outage when the indoor temperatures dropped to 45F indoors it would take an extended periods of 0F or lower outdoor temps for that pipe to actually freeze. That's just not going to happen in Boston.

But it did drop below 0F this winter for a couple of days. So much for Global Warming!
However, I probably could form a small polyiso box around the piping, keeping it to the warm inside.

If you wanted to you could skip the polyiso under the subfloor and just air seal the subfloor to the framing and it would still meet code.

Can you elaborate what you mean by air seal. Should I skip the polyiso, or does that give me benefits of a warmer upper floor on the cantilever?

There appears to be a thermal bridge where the joists go over the 2x4 & brick wall. A cut'n' cobbled slab o' foam that extended over the top plate of the 2x4 & brick foam-sealed to the joists would be a good idea, along with moving your polyiso air barriers toward the interior, so they are roughly co-planar with the wallboard of the studwall. You'd then stuff the R30s so that they extend over the top of that polyiso slab over bricks & studwall filling the cavity.

Look at my revised image, which is pretty much to scale, and I think I have it as you describe.

As drawn there is no foundation insulation either, which is another major heat leak. A typical 8-10" poured foundation is about R1. .............But for the sub grade section it's critical that there be no interior side vapor retarders (latex paint on wallboard is fine, but not vinyl or foil wallpapers.)

The brick has weep hole gaps that a bumble bee goes into. I assume that's in the air gap between brick and the black vinyl. I was thinking of plugging the weep holes up with copper wool or similar.

The bottom floor is finished, so I can't be sure of the inside wall construction, but there definitely is a 2x4 studding on the inside cement foundation (because there are wall sockets along there and also more hot water pipes going through the studs near the bottom of the floor) with some insulation (but who knows how much and how well!) The outside concrete foundation is either at grade, or at most, 18" below grade in reference to the bottom floor. My question about that is what keeps the warm bottom floor air from condensing on the cold concrete foundation and evaporating, even if it is covered by some insulation and sheetrock?

Unrelated to this, but interesting, is the contractor who built the house ran hot and cold water pipes for a downstairs bathroom to a sink from several feet inside the house to against the outside wall, and then back inside to over the sink area (several feet in and down). Besides wasting extra copper, what kind of idiot would run pipes close to the edge of the house unless absolutely necessary? If I redo that bathroom, that's the 1st thing that gets changed.

Thanks so much for your help. You have been extremely illuminating!

 

[Dana]

Don't out & out plug the weep holes or you'll have a mold-farm on your hands. The black stuff is not vinyl, it's a semi-permeable (probably asphalted or rosin loaded) product. You may be able to simply glue some screen mesh over them. If you're more ambitious you could even core them out with a masonry hole saw and install a round screened vent, eg: http://www.roundvents.com/Aluminum-Open-Screen-vents-Tab-style_c12.htm which would improve drying rates.

The bottom air barrier doesn't need to be anything special- it's never going to get direct wetting from rain, and has ample drying capacity toward the exterior. In fact, if the subfloor above is plywood and air tight it doesn't even need the cut'n'cobbled foam at the subfloor, but it still needs it between the joists and over the top of the brick & pony wall framing. The cantilever will dry toward the exterior just fine through standard half-inch OSB/plywood.

Condensation/adsorption occurs when the temperature of the surface is at the dew point of the proximate air. Concrete & wood take on moisture as adsorb (neither liquid nor vapor) fairly readily. For concrete it's not a problem- there are bridges that have been supported by concrete footings under water for the past 2000 years without failing in Europe. But for wood it is. Any time the moisture content is over 25% mold & fungus growth grows exponentially. Moisture that finds it's way into the concrete is distributed via capillary action, and can dry toward the exterior where it's above grade. Most bricks have comparable capillary draw, so the fact that the brick is above grade gives the foundation a drying path toward the exterior, even where there is effectively zero above grade exposure on the concrete itself.

Below grade you also have groundwater issues to be concerned about, which is an even bigger issue than wintertime moisture getting into the concrete. Most homes don't have a capillary break between the footing, even though the exterior has (usually) been waterproofed, and moisture wicking up the foundation from wet footings is a common failure mechanism when insulated studwalls are placed against the foundation. Installing interior vapor barriers (foil or plastic) on the interior of the stud wall makes the ground moisture problem even worse. The solution in better new construction is to install sufficient air-impermeable (usually rigid foam) insulation between the studwall and the foundation to limit wintertime condensation issues in the above grade section and leaving the interior side of the studwall relatively vapor open (latex paint, at about 3-5 perms, not vinyl wallpaper.) If those studwalls don't have mold issues by now, it means you're probably going to be fine forever, even if it's a sub-optimal stackup. It means the footings & walls are sufficiently well drained, and the drying rate of the foundation toward the above-grade exterior is managing most of the wintertime burden, keeping the cold edges of the studs sufficiently dry to not rot. It's possible it was built with pressure treated 2x4s , which would also limit the mold hazard, if only due to the arsenic content.)

A 2x4/R13 wall has a "whole wall" performance equivalent to about R10 or slightly better after factoring in the thermal bridging of the framing, which is below current code minimums, but not worth fixing until you're ready to remodel. In any re-build an inch of polyiso trapped to the wall with a 2x4/R13-R15 cavity fill would meet code performance, and would not have wintertime condensation issues where it's above grade.

 

[miracj]

Don't out & out plug the weep holes or you'll have a mold-farm on your hands.

I was told to use copper or stainless steel wool (non rusting) to plug the holes (although if it was touching another metal, it could cause some galvanic action between the metals), but to still allow the moisture out. Obviously plugging the holes so there is no moisture outlet would be real bad! There is also something similar used in roof ridge vents that is black and like steel wool but is a fiber material, that also might be usable, but would not rust. But I was told that rodents don't like the metallic wools. I wonder if a small piece of window screen pushed in a bit would also be a good choice and allow more ventilation than the wools?

Incidently the roof ridge vent on my hip roof with a 26x42 attic space was only 8 feet originally, far too short based upon my calculations, even though I never really had any issues. When I re-did the roof a year ago, I had them maximize it to 13 feet (of a 14 foot ridge), which seemed to be much closer to the Net Free Area (NFA) numbers I wanted (15.5 feet) for air flow. Again, no accolades for my builder!

If those studwalls don't have mold issues by now, it means you're probably going to be fine forever, even if it's a sub-optimal stackup. It means the footings & walls are sufficiently well drained, and the drying rate of the foundation toward the above-grade exterior is managing most of the wintertime burden, keeping the cold edges of the studs sufficiently dry to not rot. It's possible it was built with pressure treated 2x4s , which would also limit the mold hazard, if only due to the arsenic content.)

I'm sure we don't have any pressurized or other kind of treated wood (based upon studs in the garage which is one part of tbe foundation)
I know that the water table is well below us, and that our foundation is sitting on mostly granite ledge. I have never noticed any kind of condensation, except maybe some small paint peeling/bubbling next to the lower floor brick fireplace, but it was never wet to the touch. That may have been due to air influx around the fireplace before I caulked it.

Thanks again.

 

[Dana]

Peeling paint is a symptom of moisture "trying to get out". The source of that moisture could be from the ground, but it could also be from rain penetration of the chimney. If your basement had a significant humidity & mold problem, your nose would probably tell you.

The polypropylene roll mesh used for ridge venting would do just fine against insects. Are the holes really big enough for rodents?

The code requirements for NFA don't have a lot of science behind them, but "work-mostly" in this climate for most roofs. With a soffit to ridge venting scheme you always want the soffit NFA to be about 50% larger than the ridge vent NFA, since that reduces the net stack effect infiltration drives that might pull conditioned space air into the attic. When the ridge vent area is equal to or larger than the soffit vent area it depressurizes the attic relative to the rooms below, pulling air into the attic from conditioned space. Air sealing the ceiling plane is always important, but becomes even more so when the ridge vent is bigger than the soffit vents. Read this document for a bit more detail. (The author, Joe Lstiburek is one of the founders of Building Science Corp, and actually measures stuff, and has a track record of sometimes getting building codes amended based on the data.)

 

[miracj]

Peeling paint is a symptom of moisture "trying to get out". The source of that moisture could be from the ground, but it could also be from rain penetration of the chimney. If your basement had a significant humidity & mold problem, your nose would probably tell you.

No bad smell. My wife has the nose of a bloodhound, and could tell me in a second. It was about 6 ft high point on the lower floor, Just vinyl siding outside there and the cement foundation and no brick (except for fireplace next to it) and never seemed to continue after sealing.

Are the holes really big enough for rodents?

Not for rats, but more than enough for mice, which we did have in previous years. I don't think that those weep holes were their point of entry, but why tempt fate.

The code requirements for NFA don't have a lot of science behind them, but "work-mostly" in this climate for most roofs. With a soffit to ridge venting scheme you always want the soffit NFA to be about 50% larger than the ridge vent NFA, since that reduces the net stack effect infiltration drives that might pull conditioned space air into the attic. When the ridge vent area is equal to or larger than the soffit vent area it depressurizes the attic relative to the rooms below, pulling air into the attic from conditioned space. Air sealing the ceiling plane is always important, but becomes even more so when the ridge vent is bigger than the soffit vents. Read this document for a bit more detail.

I can always adjust the soffit venting easier than the ridge venting, and I will definitely take the research into consideration. I'm sure the builders didn't think twice about what they were doing (or maybe they did - where's that 8 foot ridge vent we had left over from the last job?)

[Edit] I think I may have read that venting document before redoing my roof. The 60/40 number sounds real familiar, so I need to go find my notes I had done back then and see what I calculated.

(The author, Joe Lstiburek is one of the founders of Building Science Corp, and actually measures stuff, and has a track record of sometimes getting building codes amended based on the data.)

Who ever heard of the idea of basing codes on data rather than fuzzy concepts?

 

[Dana]

Builders put ridge vents on all sorts of stuff where it makes no sense (like on my hip-roofed bungalow with no soffit venting.) Very few building scientists are wasting their careers as roofers or building contractors, though there are at least a few contractors out there with more than just a vague idea of what's going on (even in MA).

In a Boston location it's useful do download & refer to the Mass Save Deep Energy Retrofit Builder Guide (developed by Building Science Corporation under contract) for guidance on a lot of things when rehabbing or upgrading your house, whether you're taking it to high-R or not.

 

[Eldon62]

View attachment 33442

I've attached a picture to show my setup and my intention for the batts and solid foam. There may be space between the brick and the stud wall, but I am not sure (but would think there should be). The floor I believe is 3/4" plywood with 1/2" OSB on top of that (but I am not positive).

My intention is to use the "cut'n'cobbled Tuff-R as the air barriers where the joists cross the foundation sill" as you said, then to cover the underside of the subfloor between that piece and the Header/band Joist, and spray foam the edges to seal the . The rest of the joist would be filled with the R30 Batt, and covered by the vinyl siding underneath.

The pipes only come down through the subfloor about an inch or two from the bottom of the subfloor near the the edge of the cantilevered just before the inside stud wall and then go through the joist to go to the next hot water radiator along the outside wall.

I hope that is to code (but wouldn't surprise if it wasn't) in terms of where they drilled the joist hole.

My gut feeling that says that even if they always have hot water in them running in the winter, it would be better to cut the foam so they are on the warm side of it (which I could do).

Mine is of the same type

This is the situation.
I'm Eldon, a 54 yo Retired Air Force Engineer.
I just bought a house and am starting the process of finishing my basement.
I discovered the following design and need good advice to best fix it.
I've been following Dana and have been impressed with the quality and detail of the information.

Ok, I have a 2 foot cantilever on the front of the house (see pics)
on the exterior it is a soffit with vents attached to joists with staples.
Between the joists is old cellulose kraft faced.
There is no barrier between exterior and interior basement.
Basement is full slab with cinder block foundation walls.

I live in Chattanooga, TN Zone 4A.

Thoughts.